I think a legitimate question would be "What makes the 'purists' think they are 'purists'"? They certainly haven't travelled back in time to check their theories out! Some attempts at "purism" [ to coin a word--] I have heard in my time have come across as excruciating murder of what are potentially decent songs. In many cases it has seemed plain to me that the original song was sung in a completely different way. There seems to be a tendency among self-described "purists' to assume that old-time folks had no sense of rhythm. I tend to believe the opposite; look at the folk dancing worldwide---the kind that predates the waltzes and the soulful ballet.No lack of rhythm there! There are many songs which were intended to be sung as slow airs but not---tell Martin Carthy---too many of them for pete's sake!